1. Field of the Invention
The present invention relates to an antenna for transmitting radio signals of a first frequency and a second frequency and a multi-input multi-output (MIMO) communication device using the same, and more particularly, to a microstrip dual-band antenna including a reflector element for multiple frequency bands and MIMO communication device using a switched-beam antenna which is composed of the microstrip dual-band antenna.
2. Description of the Prior Art
Multiple-input multiple-output (MIMO) technology utilizes antenna array to receive and transmit signals, which significantly increases data throughput and coverage without additional bandwidth or transmit power, and thus plays an important part of modern wireless communication standards such as IEEE 802.11n, WiMax and 3GPP Long Term Evolution (LTE). In order to satisfy the market demand for portable communication devices, microstrip antennas (also known as printed antennas) are widely used in all kinds of portable communication devices due to merits of light weight, small size and high compatibility with various circuits.
In a MIMO communication device, dipole antennas can be preferably formed as a switched-beam antenna for realizing antenna diversity. However, dipole antennas cannot carry out high isolation and lower interference among MIMO ports since they are omni-directional. Directional Yagi-Uda antennas can be used instead. Please refer to FIG. 1, which is a schematic diagram of a microstrip Yagi-Uda antenna 10 according to the prior art. The Yagi-Uda antenna 10 consists of a driven element 100 as a dipole antenna and a reflector element 102. In another example of the Yagi-Uda antenna, at least one director element may be added in front of the driven element to increase antenna directionality and gain in the preferred direction. However, conventional Yagi-Uda antennas are mostly made for single-band systems and do not meet multi-band requirements in current multi-band MIMO communication devices.
Please refer to FIG. 2, which is a schematic diagram of a 2×2 MIMO communication device 20 according to the prior art. The MIMO communication device 20 includes a signal processing unit 200, RF transceivers 202 and 204, antennas A1-A6 in parallel and a switching circuit 206 including diodes as single-pole single-throw (SPST) switches for selecting antennas to be used to achieve desired performance. However, different number of antennas that are turned on generates different antenna impedance, which increases the complexity of impedance matching and may have an influence on transmission efficiency.
Therefore, a multi-band, switched-beam antenna is foreseen to be a key component of a multi-band MIMO communication device, e.g. an IEEE 802.11n wireless access point supporting 2.4 GHz band and 5 GHz band, and the problem resulted from using SPST switches to select antennas need to be improved.